Shear Stress Issue in Karamba3D for small lattice structures

Hello everybody,

I am working on karamba3D (Ver 1.3.3) for validating lattice structures with beams length between 1 to 10 mm and diameters between 0.5 to 1.5 mm. I encountered some computational issue in the calculation of the shear stress and the bending moment for those small lattices. In particular, their values resulted null.

To better understand the issue, I simulated a simple cantilevered beam, varying the beam length from 100 mm to 3 mm, keeping constant the diameter (1.5 mm), and applying a force of 10 Newtons. I noted that for beam lengths of 5.67 mm and lower Karamba stops calculating the shear force and the bending moment.

Could it be related to the slenderness ratio? Do you know if maybe Karmaba has a tolerance which does not allow to validate small beams?

Thank you and best regards,

Pierandrea

Hello @Pierandrea,
the reason for the behavior you encountered could be the default snapping distance Karamba3D uses for unifying nodes which is 5mm.
In order to set a different value use the ‘LDist’ input-plugs at the ‘LineToBeam’- (unfold ‘Options’ to see it) and ‘Assemble’-components.
In Karamba 2.0.0 WIP it is possible to set the physical unit of the input-geometry via the ‘Settings’-component or via the karamba.ini-file. Then you could work in mm instead of meter.
– Clemens

Hello Clemens,
thank you for your answer. I checked the LDist and it is set to 1 micron and I also tried to change it in the karamba.ini file (in version 1.3.3) but the problem still remains.
I installed the Karamba 2.0.0 WIP plug-in but it doesn’t work, almost all components give a warning of obsolescence.
I attach an example file of the cantilevered beam, you can see that below 5mm (exactly at 5.67 mm) the shear stress is neglected but the analysis gives the displacement result.

Pierandrea

Hi, there is no file attached.

test.gh (36.9 KB)
Hello Matthew,
sorry my mistake, here we are.

Hello @Pierandrea,
there is a bug which causes the zero cross section forces results. I will try to solve that. When setting the basic input to mm (see test_cp.gh (31.4 KB)) one gets results but there is still a conversion error concerning the cross section size. i will try to fix that as well.
– Clemens

Hello Clemens,
Thank you for your answer, I’m looking forward to your response.
Anyway, Karamba 2.0.0 WIP does not work in my Rhino 7.

Pierandrea

Hello @Pierandrea,
what error message do you get under Rhino 7?
Try to update Rhino7 to the current build - it should work then.

  • Clemens

Hello Clemens,
I attached some screenshots of the errors, I update Rhino7 but still, it does not work.

Pierandrea


Dear @Pierandrea,
try to uninstall Karamba3D for Rhino 7 manually, then reinstall it.
I found the bug which led to zero displacements and cross section forces in case of small structures. It will be published within the next fey days on Github.
– Clemens